This invention relates to a cyclone.
This invention relates particularly, but not exclusively, to dense medium cyclones for treating fine particles and classification cyclones and it will be convenient hereinafter to describe the invention with reference to these example applications. However, it is to be clearly understood that the invention is capable of application to other cyclones for example, water washing cyclones and dense medium cyclones for treating coarse particles.
In the specification the term xe2x80x9ccyclonexe2x80x9d is to be interpreted broadly and specifically to include hydrocyclones. Hydrocyclones which treat liquids containing entrained particles are thus a subset of the term xe2x80x9ccyclonesxe2x80x9d.
Broadly, a cyclone comprises a body defining an interior space having an upper cylindrical portion and a lower frusto conical portion. Fluid having entrained particles enters via a tangential or involute inlet towards the upper end of the body and passes out through an axial underflow outlet towards the bottom end of the body. Fluid and particles which do not pass out through the underflow outlet travel upwardly in an air core through a central region of the interior space of the cyclone and out through the overflow outlet. The overflow outlet is formed by a vortex finder which projects in through the top of the body of the cyclone into the interior space.
The shape of the body of the cyclone induces a helical spiral flow of fluid in the body in a radially outer region of the interior space. Then the flow changes direction and an air core spirals upwardly through a radially inner region of the interior space of the cyclone. The spiralling flow of fluid applies a centrifugal force to particles entrained within the fluid and exerts differing forces on the particles depending on their size and/or specific gravity. Heavier and/or larger particles are radially displaced towards the radially outer region of the interior space from where they pass out through the underflow outlet. Lighter and/or smaller particles tend to gravitate towards a radially inner region of the interior space and are carried upwardly with the air core which flows out through the overflow outlet. This thereby effects a separation of particles on the basis of specific gravity or size which is the key function of the apparatus.
Dense media cyclones are used for beneficiation of mineral ores, e.g. separation of heavy minerals or coal from unwanted gangue or tailings on the basis of difference in specific gravity. One application of dense media cyclones is to separate coal from non coal material in the ore which is mined out of the ground. In dense media cyclones, the relatively light coal particles are predominantly carried with the dense medium or liquid through the overflow outlet while the relatively heavier non coal particles are predominantly passed out through the underflow outlet. The efficiency of the cyclone is measured by its ability to provide a relatively sharp separation of coal and non coal particles, e.g. to reduce contamination of waste materials in the product and to reduce loss of valuable product with the waste materials.
A known prior art dense media cyclone is the cyclone illustrated in FIG. 1. This cyclone has a side wall comprising an upper wall portion of circular cylindrical configuration and a lower wall portion defining an interior space. One part of the lower wall portion has a frusto-conical configuration and the other part is in the form of a spigot projecting outwardly away from the end of the frusto-conical part. The cyclone has a vortex finder which has a relatively thin wall and does not occupy more than one third of the cross sectional area of the interior space of the body of the cyclone adjacent the inlet. Thus, the annular cross sectional area for fluid flow defined between the vortex finder and the upper wall portion is fairly large and the velocity of the fluid drops when it enters the cyclone. Another feature of the prior art dense media cyclone is that the internal junction between the frusto-conical part and the spigot part of the lower wall portion is smooth and without any interruption.
One limitation of these prior art cyclones is that there is a tendency for fluid to short circuit from the inlet to the overflow outlet without being subjected to the vigorous centrifugal forces developed in the interior space. There is also a tendency for fluid to short circuit along the side wall of the body or to be positioned in a boundary layer adjacent the side wall. Further there is a tendency for heavier particles to be entrained in an axial air core flowing up from the underflow outlet to the overflow outlet if the air core is too close to the side wall of the cyclone. The air core is very unstable. These occurrences detract from the efficiency of these cyclones.
Another limitation of the cyclone is its ability to separate out fine particles, eg 0.5 mm to 0.1 mm. This is because of the limited amount of centrifugal force generated in the cyclone. Currently, separation of very fine heavy minerals (2 mm to 0.1 mm) is undertaken using gravity concentrators, such as shaking tables, spirals and diaphragm jigs. However, the separation efficiency of this equipment is typically quite low. The concentrate from the separation often contains a considerable amount of waste material and a further separation, e.g. a downstream heavy liquids process, is often required.
Clearly therefore it would be advantageous if a dense media cyclone having increased efficiency and reduced contamination of waste material and product could be devised. It would also be advantageous if a dense media cyclone could be devised with an increased ability to separate out fine particles and thereby obviate the need for a separate step.
Classification cyclones are widely used in many industries for a variety of tasks in liquid-solid separation, including classifying, thickening, clarification, and desliming. Conventional classification cyclones have similar structural features to the dense media cyclone described above, but with different dimensions.
FIG. 2 is a schematic illustration of a conventional classification cyclone.
During operation of the cyclone, the spiralling flow and resultant centrifugal force, causes solids to be flung to the lower wall portion and spiral down to the underflow outlet. The bulk of the fluid e.g. liquid and very fine particles spiral upwards and leave the cyclone through the overflow outlet.
For any inlet pressure and rotational speed there is a theoretical xe2x80x9ccutxe2x80x9d size at which the centrifugal and drag forces are in balance. Theoretically, particles finer than the cut size are dragged with the bulk of the liquid through the vortex finder, and particles coarser than the cut size report to the underflow outlet.
Currently, the performance of conventional classification cyclones in industry is most unsatisfactory and contamination of particle sizes occurs in both the underflow and overflow material.
The reasons for this are varied and include the reasons articulated above in relation to the dense media separation cyclones. Some efforts have been directed towards the following areas in an effort to reduce these problems: change of feed parameters; modification of underflow and overflow outlets; introduction of additional components or formations to alter fluid-flow patterns; modification of the shape of the lower wall portion; and multi-stage cyclones. However, none have been commercially successful, mainly due to cost.
It would therefore be advantageous if a classification cyclone, e.g. a hydrocyclone, could be devised which at least partially overcame these problems.
According to one aspect of this invention there is provided a cyclone for effecting a separation on a fluid stream containing entrained particles, the cyclone including:
a body having a circumferential side wall extending between upper and lower ends and defining an interior space, the side wall comprising an upper wall portion and adjacent lower wall portion tapering inwardly in a direction away from the upper wall portion, the upper wall portion defining an inlet for introducing fluid and entrained particles into the interior space and the lower wall portion defining an underflow outlet extending in the direction of the longitudinal axis of the body for removing some fluid and entrained particles; and
a vortex finder projecting substantially axially into the interior space through the upper end of the body and terminating at an internal end positioned below the inlet, the vortex finder defining an overflow outlet which removes the remaining fluid and entrained particles from the cyclone;
wherein the vortex finder and the upper wall portion are configured to define a feed zone within the interior space of decreasing cross sectional area, extending in a direction from the inlet to the internal end of the vortex finder.
The cyclone is suitable for use as both a dense media cyclone and a classification cyclone.
Preferably, the feed zone of decreasing cross-sectional area extends the full length from the inlet to the internal end of the vortex finder.
Preferably, the vortex finder includes an outer wall which diverges outwardly in a direction towards the internal end of the vortex finder, e.g. tapers outwardly towards the internal end of the vortex finder. The vortex finder may taper outwardly at an angle of 83xc2x0 to 88xc2x0 relative to an axis extending perpendicularly to the longitudinal axis of the body. For a dense medium cyclone, this angle is preferably 83xc2x0-87xc2x0 and for a classification cyclone this angle is preferably 84xc2x0-88xc2x0.
The outer wall may diverge outwardly with a configuration other than a taper. For example, the outer wall may also be of undulating or stepped form.
Thus, by decreasing the cross sectional area of the feed zone in the annular space between the upper wall portion and the vortex finder, the velocity of the fluid and entrained particles entering the body is increased. This leads to increased centrifugal forces being applied to the particles driving the heavier or larger particles towards radial positions proximate to the side wall of the body and away from the vortex finder.
Further, by having the cross sectional area progressively decreasing in a direction away from the inlet, the fluid is accelerated and the centrifugal forces progressively increased thereby exerting a progressively increasing influence on the particles. This reduces the propensity of the heavier particles to short circuit directly to the overflow outlet.
In addition to assisting and increasing the centrifugal forces as described above, the outward taper on the vortex finder directs fluid entering the body through the inlet back towards the side wall which further reduces the likelihood of short circuiting flow to the overflow outlet.
The ratio of the diameter of the wall of the vortex finder at the internal end of the vortex finder to the diameter of the aligned upper wall portion of the body may be about 0.65 to about 0.85, e.g. 0.7 to 0.8.
The diameter of the outlet defined in the vortex finder may be less than one half of the diameter of the outer wall of the vortex finder at the internal end of the vortex finder.
Thus, by having the width of the vortex finder of substantially increased diameter relative to prior art cyclones, the cross sectional area for fluid flow is substantially decreased increasing the velocity of the fluid through this region of the interior space of the cyclone.
The thickness of the outer wall of the vortex finder may be 17%-23% of the diameter of the body of the cyclone at a position aligned with the internal end of the vortex finder, e.g. 17% to 20%.
Typically, the upper wall portion tapers inwardly in an axial direction away from the inlet, e.g. at an angle of 2xc2x0 to 25xc2x0 relative to the longitudinal axis of the body, preferably 3xc2x0 to 20xc2x0, most preferably 3xc2x0 to 10xc2x0.
Advantageously, the upper wall portion of a dense medium cyclone tapers inwardly at an angle of 8xc2x0 to 10xc2x0. Advantageously, the upper wall portion of a classification cyclone tapers inwardly at an angle of 3xc2x0 to 5xc2x0.
Typically, the lower wall portion tapers inwardly away from the upper wall portion at an angle of 2xc2x0 to 12xc2x0 relative to the longitudinal axis of the body, preferably 4xc2x0 to 10xc2x0, most preferably 4xc2x0 to 6xc2x0.
According to another aspect of this invention, there is provided a cyclone for effecting a separation on a fluid stream containing entrained particles, the cyclone including:
a body having a circumferential side wall extending between upper and lower ends and defining an interior space, the side wall comprising an upper wall portion and adjacent lower wall portion tapering inwardly in a direction away from the upper wall portion, the upper wall portion defining an inlet for introducing fluid and entrained particles into the interior space and the lower wall portion defining an underflow outlet extending in the direction of the longitudinal axis of the body for removing some fluid and entrained particles; and
a vortex finder projecting substantially axially into the interior space through the upper end of the body and terminating at an internal end positioned below the inlet, the vortex finder defining an overflow outlet which removes the remaining fluid and entrained particles from the cyclone;
wherein the vortex finder occupies at least 40% of the cross sectional area of the body of the cyclone at a position aligned with the internal end of the cyclone.
Typically, the vortex finder occupies between 40% and 60 of the cross sectional area of the body of the cyclone at a position aligned with the internal end, more preferably between 40% and 55% of the cross sectional area.
According to yet another aspect of this invention, there is provided a cyclone for effecting a separation on a fluid stream containing entrained particles, the cyclone including:
a body having a circumferential side wall extending between upper and lower ends and defining an interior space, the side wall comprising an upper wall portion and adjacent lower wall portion tapering inwardly in a direction away from the upper wall portion, the upper wall portion defining an inlet for introducing fluid and entrained particles into the interior space and the lower wall portion defining an underflow outlet extending in the direction of the longitudinal axis of the body for removing some fluid and entrained particles; and
a vortex finder projecting substantially axially into the interior space through the upper end of the body and terminating at an internal end positioned below the inlet, the vortex finder defining an overflow outlet which removes the remaining fluid and entrained particles from the cyclone;
wherein the lower wall portion defines a formation projecting inwardly into the interior space of the body.
Preferably, the formation is a shoulder extending substantially fully around the circumference of the lower wall portion, e.g. having a depth of 1 mm to 5 mm and/or 3%-6% of the diameter of the underflow outlet.
Typically, the lower wall portion forms a spigot adjacent the lower end of the body and the shoulder is formed proximate the spigot.
The shoulder has the effect of spacing the side wall of the body away from an air core or air plug which flows through the underflow outlet of the body and up through a central region of the interior space and out through the overflow outlet. By spacing the side wall away from the air core, the air core is less inclined to entrain heavier particles in a radially outer position and sweep them up and out through the overflow outlet.
In a cyclone of up to 100 mm diameter, the shoulder preferably has a depth of about 1 mm. The depth of the shoulder is dependent on the particle size to be separated and the underflow diameter.
The invention also extends to a cyclone having a body and a vortex finder, e.g. as described above, and wherein the outer wall of the vortex finder tapers outwardly towards the internal end of the vortex finder, e.g. at an angle of 83xc2x0 to 88xc2x0, to an axis extending perpendicular to the longitudinal axis through the body of the cyclone.